The present invention relates generally to shift control systems for automatic transmissions, and more particularly to a shift control system for an automatic transmission, configured to suitably control shift operation requested just after the engine being restarted after a period of rest, so as to reduce a potential shift shock.
In general, leaving an internal combustion engine at rest for some period of time leads to dropping down to an oil pan hydraulic fluid such as lubricating oil with which a hydraulic circuit is filled by an oil pump driven by the engine. This is true for hydraulic fluid in a hydraulic circuit that supplies hydraulic pressure to frictional engaging elements such as a hydraulic clutch and a hydraulic brake in an automatic transmission. When the hydraulic fluid in the frictional engaging elements and in the hydraulic circuit drops down into the oil pan, air enters the frictional engaging elements and the hydraulic circuit. During shift operations just after restarting the engine, the entrance of air results in a delay in engagement of the frictional engaging elements and thereby in a delay in shift response. In addition, this delay tends to increase a shift shock.
A Japanese Laid-open Patent Application No. H8(1996)-166058 shows a technique against such a problem as discussed above. In this technique, first, an engine rest time period Tstop is estimated based on an oil temperature at the last engine stop T1, an oil temperature at the restart of the engine T2, and an atmosphere temperature T3. In case engine rest time period Tstop is greater than or equal to a predetermined threshold time period Ttrg, it is determined that air enters a hydraulic circuit for supplying hydraulic fluid to a frictional engaging element. In response to the determination, the line pressure and the accumulator back pressure are increasingly adjusted during shift operations in a predetermined number of times just after the engine restart, to avoid the delay in engagement of the frictional engaging elements.